Universal focus sound scanning system



Deg. 29, 1953 F. N. GILLETTE 2,664,470

UNIVERSAL FOCUS SOUND SCANNING SYSTEM Filed May l 5, 1952 2 Sheets-Sheet1 Dec. 29, 1953 F. N. GILLETTE UNIVERSAL FOCUS SOUND SCANNING SYSTEM 2Sheets-Sheet 2 Filed May 15, 1952 QQQN Patented Dec. 29, 1953 UNIVERSALFOCUS SOUND SCANNING SYSTE Frank N. Gillette, Pleasantville, N. Y.,assignor to General Precision Laboratory Incorporated, a corporation ofNew York Application May 15, 1952, Serial No. 288,079

Claims. (01. 179-10031) This invention pertains to systems forreproducing sound modulations which have been photographicallyrecorded'on film strips and more particularly to a system which requiresno manipulation or adjustment to faithfully reproduce the recorded soundregardless of the relative surface orientation of the film with respectto the system. In reproducing sound recorded film in the projection ofmotion picture sound film and the like, a narrow band of film near oneedge is utilized as the sound track, the sound vibrations having beenrecorded thereon either as a variable density or as a variable widthtrack. In order to secure reasonable efficiency of reproduction of thehigher sound frequencies the light of an exciter lamp is projectedthrough a narrow slit, the illuminated image of which is sharply focusedon the film sound track. The permitted error in focus is in fact, muchless than the film thickness of .006 inch.

A phototubepositioned on the side of the film strip opposite to thatoccupied by the exciter lamp, scanning slit and associated opticalequipment converts the variable amount of light transmitted through thefilm strip to proportional electrical variations which in turn areconverted to sound variations in a well-known manner.

In projecting 16 mm. film incorporating a sound track, there is atpresent no standard as respects the position of the emulsion surface ofthe film. That is to say, the emulsion surface may be positioned eithertoward or away from the phototube. For present purposes a film which atthe sound drum has its emulsion toward the phototube will be termed ofthe front-surface type, while film whose emulsion surface faces awayfrom the phototube will be termed of the rear-surface type. Since bothtypes of 16 mm. film are in existence, it is necessary in general for a16 mm. projector to be designed to handle either type of film, and inorder to do this the projector must be capable of focusing the lightimage of the-scanning slit on either surface of the film. Ordinarilywhen changing from 'one type of film to the other the refocusing of thescanning slit image must be ac- ,complished by a readjustment of thescanning optics or other similar manually manipulative procedure.

The present invention eliminates this manual operation by providing anoptical system-that simultaneously has two foci, focusing the imagenecessary for him to know which type of film he is using. In utilizingthe present invention the sound recordings on both types of film arereproduced equally well, the quality of reproduction being equal to thatof present sound repro ducing systems requiring adjustment. 7

The principal object then of this invention, is

to provide a motion picture projector requiring no adjustment or changeto reproduce recorded sound from either front-surface or rear-surfacefilm.

More generally, an object of this invention is to provide a device forthe automatic reproduction of sound from front-surface film orrear-surface film with equal quality.

A further understanding of this invention may be secured from thedetailed description and accompanying drawings, in which:

Figure 1 is a diagrammatic illustration of an optical system employingthis invention.

Figure 2 is a view of certain optical details looking in the directionof line 2-2 of Fig. 1.

Figures 3 and 4 are diagrammatic illustrations of parts of the opticalsystem of Fig. 3, changed in size and proportion for convenience inanalysis.

Figure 5 schematically depicts a representative optical system. v

Figures 6 and 7 are enlarged edge and plan views respectively, of anilluminated film sound track.

Figure 8 depicts characteristic loss-frequency curves of the soundscanning system of the invention.

The well-known stereopticon type of sound reproduction optical system ischosen for illustration of the principle of this invention, althoughapplication is not restricted to this type but includes the motionpicture optical type, the cylindrical type, and all other types ofsystem for scanning a sound record on film by a focused beam of light.

Referring now to Fig. 1, a condenser lens ll focuses the light of acoiled lamp filament l2 at a point ,fc. A scanning slit I3 is formed byproviding an aperture in an opaque sheet I4, the slit being .002 inchwide and 0.268 long. The slit I3 is positioned so that its centercoincides with the of the scanning slit with the requisite sharpness eon both surfaces of film at one and the same time. The operator of theprojector therefore need make no adjustment in changing fromfront-surface film to rear-surface filmland, indeed, it is not opticalaxis of the system and is located between the condenser lens H and anobjective lens [6. The objective lens i6 is designed and positioned soas to have its normal conjugate foci at the slit [3 and at the near orrear surface I! of 16 mm. film [8. A phototube I9 is placed on the otherside of film l8 in position to be actuated by light transmitted throughthe film.

I Additionally a transparent optical glass plate 2| is positionedclosely adjacent the objective lens (6 between the lens [6 and the slitI3 with one edge thereof coinciding with the optical axis of the system.Such an arrangement, as illustrated in Figs. 1 and'2, results in theinterception of one-half of the cone of illumination 20 by the opticalplate 2l while permitting the other half thereof to be transmittedunimpeded towards the film strip l8. The positioning of the plate 2|distant from either focal point causes it to have equal eifect upon allrays of light arriving at the receiving focal point. In this respect theglass plate can be placed equally well u on either side of the objectivelens 16, but its position on the light source side ispreferable'becau'se. it results in a thicker and more convenient glassthickness.

7 When light is transmitted from lamp I2 through lenses H and i6, andthe optical plate is removed, an image of the slit I3 is projectedantl'sharply focused on the rear side I! of the film 18fthe opticaldimensions being such as to reduce the slit image dimensions at the filmwidth of .0005 inch and a length of .072 inch transverse tofthe film. Asthe film moves uniformly past the light beam at the normal rate for 16mm. film of "7.2 inches per second, the variable density of its soundtrack modulates the light passing through it, and th modulated lightfalls on the cathode of the photot-ube 19. Conventional phototubecircuits 22 amplify the modulated phototube current and apply it to thespeaker 2?).

When an interrupting plate of any kind is interposed in one-half of theconverging light beam between the slit [3 and the objective lens 16, theplate being distant from both focal points, its effect is uniform uponall parts of the image as before stated and causes a reduction of thesharply-focused image illumination by one half. The plate such as 2| hasthis effect "even though transparent because, sinc'eits index ofrefraction i'sjgreater than that of air, the focus of those raysintercepted thereby lies at 'a point more distant than thesurface l I.

The exact point at which these intercepted rays arefocujseduepends up'on'the angleof divergence 'ofthe 'rays'at the plate, the index ofrefraction of the optical glass plate, the plate thickness and if, "asher'e,"'p'art of the light extends through the iilmfthe length of thisfilm path and the film's index of refraction. From these data the properglass thickness is computed to cause the light image transmitted throughit to fall on the front surface 24 of the film iii.

In "Fig. 3 the "glass plate 21 and theincident and refracted ray areredrawn to a larger scale. In'the index of refractionfor this glass isN, then definition holds for both convergent and dii fgeil b incidlitlight, a. and at being the angles 4 Dividing (3) by (2) tan a' 1; tan a;t (4) sin 0: cos a 1 t sin :2 cos a (5) which reduces to N1 (50s a p t Ncos a (6) At the optical axis a and 0!. become zero, and in Fig. 1 theyare small and nearly equal. Therefore (6) can be simplified to Infocusing on the film front surface a similar refraction occurs, as shownin Fig. 4, so that in terms of film thickness e and film index ofrefraction n, the axial refractive shift 17' is As an example, for filmhaving the standard thickness of .006 inch and the usual index of 1.500,p=.002 inch. If s is the axial shift required of the optical system toshift the focus from the rear surface to the front surface of the film,it is seen from Fig. 4 to equal s=ep (9) The optical system of Fig. l isredrawn schematically in Fig. '5 with the principal foci Fand Findicated at the equal focal distances 1 and i. For clarity a thin lenshaving a single principal plane is indicated. The object and image havehalf-diameters A and B, so that by definition where m is magnification.

In the triangles abF, Fcd, dgh and hilc,

It is evident that such thickness t of glass 21 is required that whensubstituted in Equation '1 that value of p results which will in turncause a shift s of the right-hand focalplane. Accordingly, substitutingthe values "for a: and :02 secured from Equations 11 and 13 "in Equation14,

" t d-W] Substituting for'p its value from (7) and solving for t,

aeeigvo Substituting for s its value and for :29 its value from (12) in(16), and clearing,

As an example of the use of this equation, employing the opticalconstants of one wellknown projector, the glass thickness is computed tobe 0.16 inch.

The images so focused upon the front and rear surfaces are ofpractically equal brightness, since the glass plate intercepts exactlyone-half of the beam. However, this condition is by no means essentialto the invention and any desired degree of interception by the glass ofthe beam may be employed. I

Surrounding the sharply focused image on the rear surface I? of the filmthere is, of course,a zone of defocused light of lower brightnessresulting from the portion'of the beam that is focused on the frontsurface 24. Similarly and for a similar reason, the front surface imageis also surrounded by a defocusedzone of lower brightness.

In Figs. 6 and '7 the film I8 is illustrated as having a thickness e.One sharply focused beam 26 produces a slit image of width 10 on therear surface I1, and the other sharply focused beam 12'! produces a slitimage of similar width to on the front surface 24. If the beam halfangle is a, the low-light width (2 on the one side of the rearsurfaceimage is equal to the film thickness multiplied by tan a, or

(1:6 tan a. (19) The sum of the zones on the two sides is then 2d or 2etan a, and the total width T is 2d+w or T=w+2e tan a (20) dbattenuation=20 log (21) in which 2 is the frequency and 'u is the filmvelocity. This equation is plotted as curve A of Fig. 8 for standard 16mm. film speed and a slit image width of .0005 inch.

In accordance with this equation, that half of the light energy which issharply focused on the rear surface of rear-surface film is modulated inaccordance with the variations of the sound track, and passes throughthe sound track to actuate the phototube. The actuation of thephototube, however, departs from fidelity by reason of the finite widthof the slit image, so that the sound track frequencies reproduced in thephototube current suffer the attenuation represented by curve A.

A second curve B depicts attenuation when the slit width is .0035 inch,and represents the frequency characteristic of the defocused lightcovering the distance T in Fig. 6. Since the total quantity of thisdefocused light is equal to the total quantity of focused light, thecomparable curve B starts at the same point on the db scale as curve -A.Curve 0 isthe sum of curves A and B and represents the total effect onthe phototube of both the focused and defocused light modulated byrear-surface or non-standard film. This curve indicates that the useofdual focal planes causes the low frequencies to-be emphasized by amaximum of 6 db. This effect, however, is easily neutralized by adding aconventional compensating network to the amplifier 22, Fig. 1. Thiscompensating circuit has a frequency characteristic at frequencies below2055 cps. that is the inverse of that part of curve C, so that theresulting audio power applied to the speaker has a fiat characteristicsimilar to the lower part of curve A. The increase" of flowerfrequencies in standard orfront'surface' projection depends on thedistance T, Fig. 6, whichis slightly less than T, and

has the value I T' =2c tan a--w (22) This frequency relationship addedto that of curve A is plotted as curve D, and the compensating networkis preferably designed as a compromise based on the average of curves Cand D.

Thus the interception of substantially half of the light projectedthrough the optical lens system It by an optical plate 2| of suitablethickness results in producing a focused scanning slit image at both thefront and back surfaces of the film strip so that regardless of thesurface on which the sound is recorded faithful reproduction thereof isattained.

What is claimed is:

1. A sound scanning system for reproducing sound modulationsphotographically recorded on the sound track of a film strip comprising,a scanning slit, means for illuminating said scanning slit, means forfocusing an illuminated image of said scanning slit on one surface ofsaid film strip, a transparent plate interposed between said scanningslit and said film strip said transparent plate being positioned so asto intercept a portion only of the light transmitted from said focusingmeans to said film strip, and means for converting light variationstransmitted through said film strip into corresponding electricalvariations.

2. .A sound scanning system for reproducing sound modulationsphotographically recorded on the sound track of a film strip comprising,means for focusing a narrow band of light extending across the soundtrack on the surface of said film strip adjacent said focusing means. atransparent plate positioned to intercept a portion only of the lighttransmitted by said focusing means, said transparent plate beingpositioned adjacent said focusing means, and means for converting lightvariations transmitted through said film strip into correspondingelectrical variations.

3. A sound scanning system for reproducing sound modulationsphotographically recorded on the sound track of a film strip comprising,a source of illumination, an optical system for converting light derivedfrom said source to a narrow sharply focused band of light impinging onthe surface of said film strip adjacent said optical system andextending transversely of said sound track, a transparent plate having astraight edge thereof normal to and coinciding with the axis of saidoptical system, positioned in the light path of said optical system at adistance from said film strip and means for converting light variationstransmitted through said film strip into corresponding electricalvariations.

,csil c teetione ts i l EEK-@QSJEQF deriv j e e s seux te. a narr wShel-ply focus'd d of light impinging bfithe, surface of saig filtnstllif) ticljacent said optical system and extending tmnsyetse ly' of saidsound 5 track, a transparent plate having a straight edge .e hsljfi imante is @5 5 in ed i e axi of mitted through saidefilmstrvinmtom we 5ai H i= 1 t1 pesl oe d n" h t. P electrical variations, and eatanspalqent plate p95 of optical systen; at a. distance from said filmsitioned to inter-Giant aepq -tion ,Onlx'lqf 'th lignt strip, saidtransparent plate having" a thickness transmitted by said mousi mean e te t expressedhy .thenegue ivp parent plate being positioned; adjegentsaii N mze *Nf f WW1 Sound cann n -system- FQL'JQQEQQ!) 35 soundmodulations photographically re o dc the sound track of a. film stgip c9lnp tt ising for focusing a narpow band Of; ligh ext across the soundtrackpn tn surf i strip, means 01 conyexting l g nt er ejn inemeans h vn a thickne messes-J95 the equation N' ne 15 in which is the focallength of said focusing N; ffi-j me inens','N and were the-respectiveindices of'optica refraction of said plate and said film, -e isthefllmthi'ckries's and m isthe-magnification of the v mg sys jem v v v ,sto

i whiqh 1*- s tte 1 9??? film thickness and m is the magn' cation gPt1ca1system References Cited in the me of this patent -,A o dsce em sstem i9 7 r i t rt it e spungintoduletions }5h6togiephice11yfrcoidtion5133 PATENTS the. und tracker Iht'stifip dtfipristn' je Number Name Dates urce illumi e ex-.. I1. 1 v, 2: 2 15%??? M tchel May 29,1951

FRANK N. GILLETTE.

